Modern electronic imaging systems function by optically imaging the scene onto a focal plane composed of a two-dimensional array of sensors (or photosensitive sites) Each sensor collects or integrates the charge generated by a photoresponsive circuit element such as a photodiode, and digitizing that charge output to create a digital number representative of the amount of light that fell onto a particular pixel in some well defined exposure time. The array of pixels correspond to some imaged area in the scene, and therefore the two dimensional array of pixels can form an intensity map of the light levels present at each location in the scene.
In typical imaging systems, the whole photosensitive area of the imager (the film, or the array of photodiodes, or charge coupled devices) is active and exposed to light for a window of time which is common across all the elements of the active sensor area. For example, a mechanical shutter may open and close, letting the light in and then shutting it out. Alternately, an electronic signal may be used to reset the integrating components in the imager at some initial time, and a second signal may be used to shut the integration off and allow pixels to be read. This means that every signal that composes the image was captured under similar or identical circumstances. While this has many advantages, it can be a disadvantage when the scene is composed of very bright and very dim signal sources. If the exposure time is short, the dim signals will not be visible, and if the exposure time is too long, the bright signals will be saturated. If one is interested in seeing dim signals against bright backgrounds, discrimination techniques are needed, such as optical filters, to raise the effective level of the signal of interest against the signal of the background. Furthermore, imaging a modulated source which changes faster than the integration time for imaging is difficult to detect using traditional digital imaging approaches.